Semiconductor devices, such as MOSFET transistors, are generally designed to have specific performance features. For example, MOSFET transistors used as output transistors for voltage regulators or MOSFET transistors used or in other high current applications should have a low output impedance. As a result, such transistors generally have a relatively large channel width to reduce the ON impedance of the transistors. Although a large channel width is effective in providing a low output impedance, a large channel width does impose certain disadvantages. A large channel width, for example, consumes a relatively large amount of area on a semiconductor die, thus increasing the cost of devices using such wide channel MOSFET transistors. A large channel width also results in a relatively large gate capacitance, which reduces the operating speed of such MOSFET transistors.
Another parameter that affects the design of MOSFET transistors is the voltage level with which they are capable of operating. For example, the gate-to-source voltage of a MOSFET transistor must be limited to a value that does not cause damage to an insulative layer between the gate and the channel. Such gate insulation is generally in the form of a layer of silicon dioxide, which can be damaged by excessive gate-to-source voltages. The gate-to-source voltage that a MOSFET transistor is capable of withstanding can be increased by increasing the thickness of the gate insulation. However, thicker layers of insulative material can adversely affect the performance of MOSFET transistors. For example, a thicker gate insulation layer reduces the driving ability and bias current of such transistors, and it requires an increase in the length of the channels. Yet, a large channel length again increases the size that MOSFET transistors consume on a semiconductor die.
There is therefore an inevitable trade-off between the size of MOSFET transistors and their ability to handle large currents and large drive voltages. A MOSFET having a relatively small length and width could be used if the thickness of the gate insulation layer could be reduced. But reducing the gate thickness limits the voltage with which such transistors can be used. As a result, it has heretofore been necessary for MOSFET transistors used as voltage regulator output transistors or in other applications handling a high current and a high voltage to be relatively large.
There is therefore a need for a MOSFET transistor that, for example, consumes a relatively small area, can provide good drive performance and a low gate capacitance, and can handle relatively high drive voltages.